U.S. patent number 5,127,228 [Application Number 07/652,578] was granted by the patent office on 1992-07-07 for shape memory bi-directional rotary actuator.
Invention is credited to Steven R. Swenson.
United States Patent |
5,127,228 |
Swenson |
July 7, 1992 |
Shape memory bi-directional rotary actuator
Abstract
A shape memory actuator capable of high torque densities capable
of being used on space applications and other fields. The actuator
is made of two concentric tubular shape memory alloy (SMA) members
(12 and 14) torsioned along their longitudinal axis with ends
constrained relative to each other. One end of the actuator is
constrained while the other is the output. A heater (16) is located
inside the inner SMA member (12). A heater (18) is located on the
outside of the outer SMA member (14). The unconstrained end is
caused to rotate between positions by applying current to the
appropriate heater.
Inventors: |
Swenson; Steven R. (Saratoga,
CA) |
Family
ID: |
24617340 |
Appl.
No.: |
07/652,578 |
Filed: |
February 7, 1991 |
Current U.S.
Class: |
60/527;
60/528 |
Current CPC
Class: |
F03G
7/065 (20130101) |
Current International
Class: |
F03G
7/06 (20060101); F03G 007/06 () |
Field of
Search: |
;60/527,528,529 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Passive Sun Seeker/Tracker and Thermally Activated Power Module"
Seibert and Morris. .
"Shape Memory Alloy Actuator Drive Rotary Actuator" Design News,
Feb. 1990. .
"Special Alloy is Key to Braille Computer Display" Design News,
Feb. 1990..
|
Primary Examiner: Ostrager; Allen M.
Claims
I claim:
1. An actuator comprising:
a first member made of a shape memory alloy extending along a
longitudinal axis having a first and second end;
a second member made of a shape member alloy extending along a
longitudinal axis having a first and second end;
said first member being substantially tubular to accommodate the
second member in a concentric relationship within the first
member,
the members being torsioned about their longitudinal axis and
fixedly attached to each other at each of their ends;
both members being fixed to a stationary support at one of their
common ends;
a first heater in operative cooperation with the first member;
a second heater in operative cooperation with the second
member;
means for selectively activating one of said heaters to allow
rotation of the members at the end which is opposite to end affixed
to the stationary support.
2. A mechanical actuation system comprising:
a first member made of a shape member alloy extending along a
longitudinal axis having a first and second end;
a second member made of a shape member extending along a
longitudinal axis haling a first and second ends;
said first member having substantially tubular to accommodate the
second member in a concentric relationship within the first
member;
the members being torsioned about their longitudinal axis and
fixedly attached to each other at each of their ends;
both members being fixed to a stationary support at one of their
common ends;
a first heater in operative cooperation with the first member;
a second heater in operative connection with the second member;
means for selectively activating one of said heaters to allow
rotation of the members at the end which is opposite to end affixed
to the stationary support to a secondary position;
means for selectively activating the second of said heaters to
allow rotation of the members at the end which is opposite to the
end affixed to the stationary support to the initial position.
Description
BACKGROUND
1. Field of Invention
This invention relates to Machine Elements that perform rotary
functions, specifically those utilizing shape memory alloys that
reciprocate between positions.
2. Description of Prior Art
Shape Memory Alloys (SMA) form a group of metals that have
interesting mechanical properties. The property of their namesake,
shape memory, is that the alloy when deformed at below the
martensite finish temperature and then heated to above austenite
temperature, the alloy resumes its shape prior to the deformation.
Actuation devices employing SMA for light duty are well known.
Typically these actuators use a single SMA member that is deformed
in some manner and a return bias spring mechanically connected to
the shape memory member. These actuators, called bias spring type,
when heated, thermally or by other means, the SMA member returns to
shape prior to deformation and thereby overcoming the force of the
bias spring. When the device is allowed cool the spring deforms the
SMA member returning the actuator to the original position.
Actuators of the bias spring type and the differential type have
been used in micro application such as Olosky's Braille Pin
Actuator developed by TiNi Company of Oakland, Calif. to macro
applications like Sun Tracker Developed by Siebert and Morris of
Martin Merrieta Denver Aerospace, Denver, Colo.
A second type of actuator, called a differential type uses two SMA
members mechanically connected in series. Heating one of the SMA
members, performs work in one direction and deforms the other SMA
member. Heating the other SMA member performs work in the other
direction deforming the SMA member heated first.
Strange U.S. Pat. No. 4,010,455 shows a bi-directional device that
uses heat extensible springs, 24 and 26, made of shape memory
alloy. The shape memory members in this device are flat, fixed at
each end and mechanically connected to a crank to perform the
rotary actuation. The SMA members are deformed in bending and
heated by termofoil heaters bonded to the top and bottom surfaces.
Gabriel et al U.S. Pat. No. 4,700,541 shows a device with two SMA
wires 101 mechanically connected in series, torsioned along their
longitudinal axis with the ends constrained against movement. The
SMA members are heated through internal resistance with a plurity
of electrical connections. Kroll et al U.S. Pat. No. 4,887,430
shows a bi-stable device employing two opposing internally
resistance hated SMA coil springs, 16 and 24, as the SMA members. A
detent retainer 40 is used to hold the actuated element in a
desired position.
Although the Shape Memory Actuator known heretofore have worked for
their limited applications they suffer from several
disadvantages:
(a) Bias spring type actuators do not have two positions which are
stable for either the above austenite temperature or below
martensite finish temperature.
(b) Work output per unit volume is sixty percent lower for SMA
members used in bending. This is due to the neutral axis shift
caused by the difference between compressive and tensile yield
points. Most coil spring SMA members suffer this because for the
SMA member to remain in pure torsion for any appreciable motion the
spring coil diameter becomes impractically large.
(c) Internally resistance heated SMA members can be jerky in their
motion due to hot spots created by imperfections in the alloy grain
structure.
(d) Internally resistance heated SMA members are limited to small
cross sections and therefor small work outputs to be practical. As
cross sectional area increases the current required to heat the SMA
member becomes large and impractical for most applications.
(e) Solid rods in torsion are inefficient from a weight and power
standpoint. The center material of a solid rod is doing little work
for the added weight and is heated along with the rest of the SMA
member during the actuation.
(f) The differential actuators known heretofore all have each end
of the SMA members constrained with the output of the device
between the fixed ends. This makes for a device that is hard to
integrate into existing systems.
OBJECTS AND ADVANTAGES
Accordingly several objects and advantages of the present invention
are:
(a) to provide an actuator type with two positions which are stable
for both the above austenite temperature or below martensite finish
temperature condition;
(b) to provide an actuator that utilizes the material at its
maximum work output per unit volume;
(c) to provide an actuator with smooth motion between position
through even heating methods;
(d) to provide an actuator with large work outputs and practical
power requirements;
(e) to provide an actuator that is efficient in the use of the
SMA
(f) to provide an actuator constrained at one end with the output
of the other end.
Further objects and advantages are to provide actuator with
substantially higher torque densities than electro-mechanical,
pneumatic, or hydraulic actuators; to provide an actuator that
creates no noise; to provide space rated actuator at a greatly
reduced price; and to provide a more reliable actuator than the
present electro-mechanical, pneumatic, or hydraulic actuators.
Still further objects and advantages will become apparent from a
consideration of the ensuing description and drawings.
DRAWING FIGURES
In the drawing FIG. 1 shows the details of the preferred embodiment
of the invention and in FIG. 2 is an exploded view of the preferred
embodiment.
REFERENCE NUMERALS IN DRAWINGS
12--Inner SMA Member
14--Outer SMA Member
16--Inner SMA Member Heater
18--Outer SMA Member Heater
20--Indexing Standoff
22--Indexing End Cap
24--Outer Heater Switch
26--Inner Heater Switch
28--Voltage Supply
DESCRIPTION--FIG. 1 AND FIG. 2
A typical embodiment of the actuator is presented in FIG. 1 and
exploded view of that embodiment in FIG. 2. The actuator is
comprised of two concentric tubular SMA members, an inner SMA
member 12 and an outer SMA member 14. The end of the SMA members
are mechanically connected and restrained to an indexed position
relative to each other. Though not required the present preferred
embodiment uses a spline arrangement. Internal splines of indexing
standoffs 20 engage the external splines of the inner SMA member
12. The indexing end caps 22 have internal splines that engage the
external splines of both the outer SMA member 14 and the indexing
standoffs 20. The spline arrangement mechanically restrains the
ends relative to each other and provides adjustability in the
relative twist of the SMA members. Heaters can be located either on
the inside or the outside of the SMA members provide that they are
thermally insulated from heating the wrong SMA member. The
preferred embodiment has the inner SMA member heater 16 located
inside the inner SMA member 12. The outer SMA member heater 18 is
located on the outside of the outer SMA member 14.
In the preferred embodiment the SMA members are designed to provide
equal torque outputs in either direction. The inner and outer
diameters of SMA members are selected for the desired output torque
and to match the polar moments of inertia. The lengths are selected
to provide the desired rotary motion and to most closely match the
individual torque curves. We have found that a design with ten
percent strain developed in either SMA member between positions
gives satisfactory results.
OPERATION--FIG. 1
Initially both SMA members are martensite with the inner SMA member
12 twisted relative the outer SMA member 14. Actuation is
accomplished by closing the inner heater switch 26 to apply power
from the voltage supply 28 to the inner SMA member heater 16
heating the inner SMA member above its austenite finish
temperature. When the material becomes austenite the associated
shift in the modulus increases the internal stresses in the SMA
member. Strain recovery proceeds at the elevated stress until the
strain coincides with the yield point. The output torque driven by
the internal stress then drops off as function of the austenite
modulus. Simultaneous to this the outer SMA member 14 is being
deformed in its martensite form by the inner SMA member 12 which is
in its austenite form. The actuation stops when the torques of the
two SMA members balance. Reverse actuation is accomplished by
allowing the inner SMA member 12 to first cool to the martensite
finish temperature. Closing the outer heater switch 24 to apply
power from the voltage supply 28 to the outer SMA member heater 18
heating the now deformed outer SMA member 14 above the austenite
finish temperature. The actuation proceeds as it did in the
opposite direction with the roles of the inner and outer SMA member
reversed.
SUMMARY, RAMIFICATIONS, AND SCOPE
An advancement is made in the art of actuators using shape member
allows (SMA). The reader can see the simplicity of the invention
contains only six parts in the actuator not including the power
supply and switching. The actuator need only be fixed at one end
with the output taken from the other; the separate heater
arrangement provides for practical power requirements, and smooth
actuation; the concentric tube SMA members torsioned about their
longitudinal axis provide a more efficient use of the SMA than the
prior art; and the actuator being of the differential type is
inherently stable at both positions.
Further advantages are that the actuator has substantially higher
torque densities than electro-mechanical, pneumatic, or hydraulic
actuators. For example a comparison to electro-mechanical actuator
shows that the weight and volume of the present invention is only
ten percent of an electro-mechanical actuator of equal torque.
Inherent to most SMA actuators is that they create no noise. They
is advantages for marine systems and other applications where noise
is a concern. Space rated actuator at greatly reduced price can be
accomplished due to the simplicity of design and the associated
ease to manufacture. The simplicity of the designs provides for a
more reliable actuator than the present electro-mechanical,
pneumatic, or hydraulic actuators. This is crucial for space and
safety related marine applications.
It is understood that the above described embodiments and
applications are illustrative of the application of principles of
the invention. Other arrangements may be devised without departing
from the spirit and scope of the invention. For example it is clear
that an actuator with unequal torque outputs can be devised by
varying SMA members dimension to provide unequal polar moments of
inertia. In addition the cross section could be something other
than circular and still provide satisfactory results.
Thus the scope of the invention should be determined by the
appended claims and their legal equivalents, rather than the
embodiment given.
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